Terephthalic acid is produced by subjecting p-phenylene compounds such as p-alkylbenzenes, typically p-xylene, to a liquid-phase oxidation reaction in an acetic acid solvent in the presence of a catalyst such as cobalt and manganese or in the presence of such a catalyst and an accelerator such as a bromine compound and acetaldehyde. However, the reaction product contains various impurities, such as 4-carboxybenzaldehyde (4CBA) and p-toluic acid, which may cause discoloration. Therefore, higher purification techniques are required to obtain a high-purity terephthalic acid.
There are known various methods for purifying the crude terephthalic acid produced by the liquid-phase oxidation reaction, for example, a method in which the crude terephthalic acid is dissolved in an aqueous solvent at high temperatures and high pressures and the resultant solution is subjected to a catalytic hydrogenation treatment, an oxidation treatment or a recrystallization treatment, and a method in which a slurry of terephthalic acid crystals partially dissolved therein is subjected to a high-temperature immersion treatment. In particular, a method of subjecting a solution of crude terephthalic acid in water to catalytic hydrogenation in the presence of a group VIII noble metal catalyst of the periodic table at high temperatures and high pressures has been utilized for the past several tens years as a large-scale industrial process for the production of high-purity terephthalic acid.
A major problem of the catalytic hydrogenation process is that a large number of steps are required. That is, the catalytic hydrogenation process requires, as the main steps except for complicated and troublesome steps for recovery of catalyst and solvent, a series of equipments including a single- or more-stage oxidation reactor, several sequential crystallizers for rough purification, a separator for rough purification, a dryer for rough purification, a re-dissolution vessel, a catalytic hydrogenation reactor, several sequential crystallizers for fine purification, a separator for fine purification, and a dryer for fine purification.
The major factor to increase the number of steps is the use of acetic acid as the reaction solvent for the production of the crude terephthalic acid by oxidation and the use of water as the solvent for the purification by catalytic hydrogenation. To displace the mother liquor from acetic acid to water, the crude terephthalic acid produced by the oxidation must be once completely separated from the acetic acid solvent, and then dissolved again in water. If the separation of the crude terephthalic acid from acetic acid is incomplete and the crude terephthalic acid with acetic acid attached thereto is fed to the catalytic hydrogenation process, the acetic acid is mixed with water solvent for the catalytic hydrogenation and discharged out of the production system because acetic acid hardly undergoes chemical change during the catalytic hydrogenation. This means a loss of valuable acetic acid, and the discharged acetic acid must be made harmless against the environment, resulting in a large economical loss.
To eliminate the economical loss, it is required to substantially completely prevent the attached acetic acid from accompanying the crude terephthalic acid being fed into the catalytic hydrogenation process. Therefore, in the conventional industrial facilities, a combination of a separator for rough purification and a dryer for rough purification has been employed to separate the mother liquor from the crystal-containing slurry from the oxidation step. A solid bowl-type centrifugal separator or a rotary vacuum filter has been most generally used to separate the mother liquor from a crystal-containing slurry. Both the separators are widely used also in the separation of the mother liquor from the slurry containing the crude terephthalic acid crystals.
In the separation using the solid bowl-type centrifugal separator, an acetic acid slurry is introduced into a basket rotating at a high speed to centrifugally separate the crystals and the mother liquor. The mother liquor is overflowed from a weir provided on the basket, whereas the precipitated crystals are continuously scraped out by a screw. However, this method involves drawbacks of requiring complicated repair and maintenance because of its structural limitations inherent to the centrifugal separator that needs high-speed rotation. In addition, since the crude terephthalic acid crystals are separated in the form of a wet cake containing the mother liquor, an additional drying step must be provided downstream the centrifugal separation step to separate the acetic acid attached to the crude terephthalic acid crystals.
In the separation using the rotary vacuum filter, the crude terephthalic acid crystals placed on the bottom of a housing are sucked onto a cylindrical filter by evacuating the inside of the filter and move upward with the rotation of the filter. Generally, after passing through a washing zone where a rinsing liquid is sprayed over the crude terephthalic acid crystals retained on the filter by suction, the terephthalic acid crystals are separated from the filter as a filter cake. In this method, although the repair and maintenance are relatively easy because a high-speed rotation is not required, it is difficult to completely remove the mother liquor attached to the crude terephthalic acid crystals. Therefore, like the separation using the centrifugal separator, this method also needs a downstream drying step.
To solve the above problems, there have been proposed methods capable of more efficiently removing the mother liquor from the crystals, for example, a method using a separator equipped with a movable filter band (for example, JP 5-65246 A) and a method using a pressure-type rotary filtration separator (for example, JP 6-502653 A). In these methods, since the separated crystals are washed with water to displace the attached mother liquor (acetic acid) by water, a dryer is not required. Although a dryer is not required, these methods require separators of more complicated structure, this making these methods not so advantageous for simplifying the process.
To simplify the process more, it is preferred to separate the crystals from the acetic acid solvent at a temperature close to the oxidation temperature (usually 150 to 230° C.) and feed a slurry of the separated crystals in water into the catalytic hydrogenation step (usually conducted at 250 to 300° C.). This method makes the sequential crystallizer for rough purification in addition to the dryer unnecessary and saves the energy required for cooling and re-heating the crystals and liquids. In addition, since the crystals and the mother liquor are separated at a high temperature, the amount of impurities that precipitate in the crystals from the mother liquor is reduced to increase the quality of the crude terephthalic acid crystals. This leads to another advantage of easy purification.
As a method applicable to such a process, there has been proposed a method in which a crude terephthalic acid is recrystallized from water and the resultant slurry is fed into an upper portion of a vertical column at a high temperature (165° C. or higher), thereby allowing the terephthalic acid crystals to sediment by gravity against a slow upward flow of a high-temperature water to wash away the attached mother liquor (for example, JP 33-5410 B). In this method, the separation of mother liquor is conducted at a high temperature (under pressure) after the terephthalic acid crystals are recrystallized from water. However, this method is basically a mother liquor displacement method for displacing the mother liquor of the terephthalic acid slurry by a fresh solvent.
This mother liquor displacement method is advantageous in that no special powers are required because the crystals are allowed to sediment by gravity, and is attractive because of the simple structure of the apparatus to be used. However, the method is disadvantageous in that the degree of the mother liquor separated from the crystals (hereinafter referred to as “degree of mother liquor displacement”) is low, and that experimental results are hardly scaled-up into the practical production. By increasing the speed of upward flow of the high-temperature water, the degree of mother liquor displacement can be increased. However, this requires a large amount of water. In addition, the sedimentation speed of the crystals is decreased by the increase of the speed of upward flow to allow a large amount of small-size crystals to escape from the top of vertical column.
To remove the above drawbacks, there has been proposed a mother liquor displacement method using a vertical column that is horizontally divided by a plurality of partitions with a number of holes to combine a gravitational sedimentation process of the terephthalic acid crystals and a particle transportation process (for example, JP 57-53431 A). The partitions are used to increase the degree of mother liquor displacement by preventing the channeling or back-mixing of the fluid within the apparatus. However, in the mother liquor displacement utilizing the gravitational sedimentation of slurry, the partitions cause the deposition of crystals thereon and clogging and bulking at openings to require much labor for stable operation of the apparatus.
In addition, there has been proposed a displacement column that is horizontally divided by a number of plates. The terephthalic acid crystals are scraped and allowed to fall down through small holes by a scraper blade that are rotated on the plates at a relatively slow speed (for example, JP 1-160942 A). In the working example for displacing the acetic acid solvent for crude terephthalic acid crystals by water using the proposed displacement column, a high degree of mother liquor displacement is achieved, i.e., 99.9% or more of the acetic acid solvent is displaced by water. However, in the apparatus having the fixed plates and the scraper blades rotated at a slow speed (assumed from the examples as about 0.01 m/s by a peripheral speed of the tip end of the blade), the crystals attach to the plates and scraper blades and grow thereon to make the reliability in a long-term operation poor.